Genetic Basis for Spontaneous Hybrid Genome Doubling during Allopolyploid Speciation of Common Wheat Shown by Natural Variation Analyses of the Paternal Species

<div><p>The complex process of allopolyploid speciation includes various mechanisms ranging from species crosses and hybrid genome doubling to genome alterations and the establishment of new allopolyploids as persisting natural entities. Currently, little is known about the genetic mechanisms that underlie hybrid genome doubling, despite the fact that natural allopolyploid formation is highly dependent on this phenomenon. We examined the genetic basis for the spontaneous genome doubling of triploid F₁ hybrids between the direct ancestors of allohexaploid common wheat (<i>Triticum aestivum</i> L., AABBDD genome), namely <i>Triticum</i><i>turgidum</i> L. (AABB genome) and <i>Aegilops</i><i>tauschii</i> Coss. (DD genome). An Ae. <i>tauschii</i> intraspecific lineage that is closely related to the D genome of common wheat was identified by population-based analysis. Two representative accessions, one that produces a high-genome-doubling-frequency hybrid when crossed with a <i>T</i><i>. turgidum</i> cultivar and the other that produces a low-genome-doubling-frequency hybrid with the same cultivar, were chosen from that lineage for further analyses. A series of investigations including fertility analysis, immunostaining, and quantitative trait locus (QTL) analysis showed that (1) production of functional unreduced gametes through nonreductional meiosis is an early step key to successful hybrid genome doubling, (2) first division restitution is one of the cytological mechanisms that cause meiotic nonreduction during the production of functional male unreduced gametes, and (3) six QTLs in the <i>Ae</i><i>. tauschii</i> genome, most of which likely regulate nonreductional meiosis and its subsequent gamete production processes, are involved in hybrid genome doubling. Interlineage comparisons of <i>Ae</i><i>. tauschii</i>’s ability to cause hybrid genome doubling suggested an evolutionary model for the natural variation pattern of the trait in which non-deleterious mutations in six QTLs may have important roles. The findings of this study demonstrated that the genetic mechanisms for hybrid genome doubling could be studied based on the intrinsic natural variation that exists in the parental species.</p> </div

Plot of ∆<i>K</i> from the STRUCTURE analysis of the full set of sample accessions.

<p>Plot of ∆<i>K</i> from the STRUCTURE analysis of the full set of sample accessions.</p

Pollen mother cell observations.

<p>Stages of normal nonreductional meiosis (A–H) and PMCs undergoing atypical cell divisions (I–T) are shown. The cells in A-I, L, N and Q were sampled from the HGD hybrid, the others are from the LGD hybrid. A. Prophase. B. Late prophase. Twenty-one univalents are visible. C. Metaphase. D. Restitutive nucleus (polar view). E. Restitutive nucleus (side view). F. Anaphase. G. Telophase. H. Dyad. I. Metaphase cell having a lagging chromosome. Sister chromatids are visible. J. Metaphase cell having a lagging chromosome. Irregular cytokinesis occurs in the direction vertical to the metaphase plate (i.e., vertical cytokinesis). K. Restitutive nucleus having decondensed lagging chromosomes. L. Restitutive nucleus undergoing vertical cytokinesis. M. Dumbbell-shaped restitutive nucleus. Irregular cytokinesis forcibly splits the restitutive nucleus. N. Anaphase cell having lagging chromosomes. O. Anaphase cell undergoing tri-polar division. The irregular cytokinesis seen in J seems to have resulted in the cells of this type. P. Anaphase cell having chromosome bridges. Chromosomes separate in each of the daughter cells that are produced through irregular cytokinesis. Q. Telophase cell undergoing irregular cytokinesis. R. Pollen dyad having a decondensed lagging chromosome. S. Tetrad. T. Hexad.</p

Immunostaining of male sporogenesis of the HGD hybrid.

<p>Merged images of the chromatin (blue), alpha-tubuling (green), and histone H3 phosphorylated at Ser 10 (phosphoH3S10) (red) signals. Essentially the same immunostaining pattern was observed for the LGD hybrid. The cells shown are from the HDG hybrids. A. Early prophase. B. Late prophase. C. Early metaphase. D. Late metaphase. E. Late metaphase (polar view). F. Restitutive nucleus. G. Late restitutive nucleus. H. Late restitutive nucleous. I. Anaphase. J. Telophase.</p

Stage-wise comparisons of aberrant cell frequencies.

<p>A. Prophase. B. Late prophase. C. Metaphase. D. Restitutive nucleus. E. Anaphase. F. Telophase/Dyads. The red and blue bars respectively denote the frequencies for HGD and LGD hybrids.</p

Proportional membership (<i>Q</i>) of each <i>T. aestivum</i> accession at <i>K</i>=2.

<p>Proportional membership (<i>Q</i>) of each <i>T. aestivum</i> accession at <i>K</i>=2.</p

Hybrid genome doubling frequencies.

<p>The frequencies were measured as selfed seed set rates in the HGD hybrid (orange), the LGD hybrid (dark blue), and the segregants (green).</p

Plot of ∆<i>K</i> from the STRUCTURE analysis of the 187 <i>T. aestivum</i> accessions.

<p>Plot of ∆<i>K</i> from the STRUCTURE analysis of the 187 <i>T. aestivum</i> accessions.</p

Posterior distribution of divergence time.

<p>A. AesL1-TauL2 split. B. AesL2-TauL2 split.</p

Box plots of genome doubling frequencies of LDN-<i>Ae</i>.

<div><p><b><i>tauschii</i> triploid F₁ hybrids</b>. </p> <p>A. year 2004. B. year 2005.</p></div